In basic design, a solar cell is composed of a material such as a semiconductor substrate that absorbs energy from photons to generate electricity through the photovoltaic effect. When photons of light penetrate into the substrate, the energy is absorbed and an electron previously in a bound state is freed. The released electron and the previously occupied hole are known as charge carriers.
The substrate is generally doped with p-type and n-type impurities to create an electric field inside the solar cell, called a p-n junction. In order to use the free charge carriers to generate electricity, the electrons and holes must not recombine before they can be separated by the electrical field at the p-n junction. The charge carriers that do not recombine can then be used to power a load.
A common method for producing solar cells begins with a substrate doped to have p-type conductivity. An n-type dopant is introduced to the front surface of the substrate to form an n-type emitter layer on top of a p-type base layer. Contacts are then formed on the front surface of the emitter layer and back surface of the base layer to allow electrical connections to be made. The free electrons are collected by the front contacts and the holes are collected by the back contacts.
Since the front contacts block some of the incident sunlight, it is advantageous not to cover too much of the front surface of the emitter layer with the contact material. Instead, a traditional approach involves forming contact points, lines, or grids. Some low-cost solutions for forming these contacts, such as screen-printing, may require heavily doping the emitter layer to reduce contact resistance between the front contacts and the underlying emitter layer. The use of heavy doping, however, increases charge carrier recombination in the emitter layer and at the emitter layer surface, which leads to a reduction in overall cell efficiency.
To overcome this disadvantage, various techniques may be used to form an emitter layer that is doped relatively heavily underneath the front contacts and relatively lightly in the exposed regions between the front contacts, which is known as a selective emitter. These techniques, however, suffer from one or more drawbacks such as the need for additional steps in the manufacturing process, an increase in lifetime degradation and instability, and incompatibility with high-volume manufacturing processes.
Therefore, there is a need in the art for producing selective emitter solar cells that overcome the above-mentioned and other disadvantages and deficiencies of previous technologies.